Hollow core sandwich structures to which this invention is directed must be resistant to compression over a wide range of applied loads.
More particularly the upper sheet or skin and lower sheet or skin for such core must resist relatively great tensile and compressive forces with the core serving to stabilize the sheets or skins on those surfaces where they must act most efficiently.
The required degree of stiffness and resistance to bending and buckling and the light weight of the hollow core sandwich structure will encourage if not dictate use of such an alternative, for example in pallets where heavy loads of material or supplies or a heavy vehicle or equipment are to be transported by truck, rail or by air, weight being a significant factor in shipment costs and therefore to be minimized where possible.
One widely used core structure for pallets is balsa wood.
Balsa is lightweight and capable of accommodating heavy loads quite satisfactorily where the load is applied to the end grain.
So long as balsa is readily available and the cost reasonable, likely it will continue to be specified as core material for certain applications in the construction of pallets, base panels and other like circumstances.
Where a pallet with a balsa core is not watertight and the core becomes saturated with water and not relieved or remedied, rot sets in and the core deteriorates and no longer can the pallet handle compressive loads. Such pallet must either be repaired or rejected.
Further, with balsa cores delamination of the upper and lower sheets or skins occurs. This tendency is attributable in part to the several different coefficients of expansion, that is of the balsa itself, the metal or alloy used for the sheets or skins and the metal or alloy used in the edge formation or frame surrounding the core as well as in the adhesive used to secure or bond the components together.
Delamination in pallets using a balsa core between the top and bottom skin is a serious problem and requires frequent repair.
On the other hand a variety of low density or hollow cellular core structures have been proposed for numerous applications including pallet structures, base panels for containers and for aircraft wings.
The hollow honeycomb core configuration is common to many proposals such as appears from the pallet structure of U.S. Pat. No. 3,709,161 wherein the core is fabricated from double-faced corrugated paperboard using adhesive to bond the components together.
Alternatively, several honeycomb configurations derived from sheet units of resin impregnated glass wool are illustrated and described in U.S. Pat. No. 4,500,583.
Low density hollow metallic cores for panels that utilize a honeycomb core configuration manufactured from strip material also have been developed such as in U.S. Pat. No. 2,959,257 wherein stainless steel core strips are recommended and the composite secured to the surface sheets or skins by welding.
U.S. Pat. No. 3,017,971 likewise discloses a composite honeycomb core structure derived from corrugated metal strips to be secured by brazing, whereas in U.S. Pat. No. 3,604,374 a panel for blast protection utilizes corrugated core strips derived from steel secured to armoured steel plates by suitable adhesives.
Modified honeycomb structures serving as cores in laminated aircraft wings are revealed by U.S. Pat. No. 1,578,416 and by U.S. Pat. No. 2,720,949 each utilizing metallic strips so folded or configured as to present mating halves of hollow cells of hexagonal configuration combined with reinforcing longitudinal divider strips or ribs utilizing thermosetting adhesives or other suitable bonding agents for securing adjoining surfaces together.
A more complex combination of high density and low density honeycomb cores provided with solid spars and spar caps respectively extending spanwise of the high density core portion to serve as a strengthening or reinforcing member in airplane control surfaces such as a flap or an aileron is described in U.S. Pat. No. 4,304,376.
This structure utilizes liquid bonding adhesives to secure the core combination to the skin, requiring that the improved core components including the spar caps be precisely dimensioned before the application of the adhesive in order to avoid a rippling effect in the skin due to the character and migration of the adhesive upon curing which would adversely affect the aerodynamic properties of the airplane control surfaces.